Victims were collecting fuel from the oil spill before the truck set on fire.

At least 140 people have been killed after an oil truck burst into flames on a highway in Bahawalpur, Pakistan.

Local residents were attempting to collect fuel from the overturned truck when flames engulfed more than 100 people in the Punjab province.

Seventy-six are also reported injured and more than 40 people have been taken to two local hospitals in a critical condition.

Despite residents being told of the leaking fuel via a loudspeaker located at a local mosque, according to provincial government spokesman Malik Muhammas Ahmed Khan, "people of the area and passers by had started gathering fuel when the tanker exploded, burning everybody on and around the spot."

"People living in roadside villages, carrying domestic pots rushed to the scene to collect oil and they also made phone calls to their relatives living in other villages to immediately come to collect oil," the sources revealed.

Emergency services were investigating if speed was a factor in the original crash after the tanker flipped on a national highway, 100 kilometres southwest of Multan.

However, subsequent reports now suggest a potential burst tyre was the cause of the initial crash. The driver survived and remains in police custody.